Numerical simulation method for predicting service life of freeze-thaw concrete based on exfoliation layer depth

Abstract

The invention discloses a numerical simulation method for predicting the service life of freeze-thaw concrete based on the depth of an exfoliation layer, and relates to the technical field of concrete.The method comprises the steps that concrete material parameters are obtained through a preset rule, and a two-dimensional concrete numerical model is established; carrying out material attribute endowing on the model, deleting a damage failure part, calculating a corresponding peeling layer limit value, and carrying out grid division on the processed two-dimensional concrete numerical model; calculating a temperature field and a stress field based on the environment temperature and the grid divided model; according to the exfoliation layer limit value, the temperature field and the stress field, carrying out whole-process numerical simulation on the two-dimensional concrete numerical model after the damage failure part is deleted, and obtaining the maximum freezing and thawing times; and predicting the service life of the freeze-thaw concrete based on the maximum freeze-thaw times, the predicted regional environment temperature change amplitude and the predicted regional environment temperature change rate. According to the method, the service life of the freeze-thaw concrete is predicted.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete, and more particularly to a numerical simulation method for predicting the life of freeze-thaw concrete based on the depth of the spalled layer. Background Art

[0002] For a long time, people have generally focused on studying the bearing capacity of reinforced concrete structures. However, in extreme environments, the problems caused by insufficient durability of concrete are more severe. Especially in the cold regions of Northeast China and the cold saline environments in the Northwest of China, the durability problems of concrete are more prominent. The freeze-thaw action is a long-term and slow process. During this process, the concrete protective layer on the surface of the building continuously spalls, resulting in the exposure and corrosion of the internal steel bars. The bearing capacity of the components continuously decreases, reducing the safety and usability of the building. Seriously, it will cause certain economic losses and casualties. Therefore, to maintain the stability and long-term applicability of the concrete structure, ensuring the durability of concrete is crucial.

[0003] Under cold climate conditions, concrete structures will undergo repeated freeze-thaw cycles. When the saturation degree of the pores inside the concrete reaches 75% to 90%, the water in the pores freezes and expands, generating significant hydrostatic pressure and causing cracks in the surrounding concrete. Initially, tiny cracks may only appear between the pores or near isolated pores. However, as the freeze-thaw cycles continue, these micro-cracks gradually develop and connect with each other. Eventually, under the action of multiple freeze-thaw cycles, large areas of spalling will occur on the surface of the concrete, forming cracks extending inward. Over time, the cracks continue to expand and spread deeper, destroying the integrity and continuity of the concrete, weakening the mechanical properties of the concrete, and ultimately causing serious damage to the concrete structure.

[0004] Most of the domestic and foreign research on the freeze-thaw durability of concrete still remains under laboratory conditions, and the systematic research on the influence of long-term repeated freeze-thaw cycles on concrete structures in the actual engineering environment is still insufficient. In China, many important infrastructure facilities are located in cold regions, including some nuclear power plant facilities, road and bridge facilities, etc. These facilities are long-term facing the threat of freeze-thaw environment to their safe service performance. However, currently, the durability design of important buildings with a design service life of more than 50 years mostly relies on empirical data, and a perfect evaluation method for the freeze-thaw durability and life of concrete has not been established.

[0005] Therefore, how to predict the life of freeze-thaw concrete is an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention provides a numerical simulation method for predicting the life of freeze-thaw concrete based on the depth of the spalled layer, so as to realize the prediction of the life of freeze-thaw concrete.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A numerical simulation method for predicting the life of freeze-thaw concrete based on the depth of the spalling layer, comprising:

[0009] Obtaining concrete material parameters based on concrete materials by using preset rules, and establishing a two-dimensional concrete numerical model through the concrete material parameters;

[0010] Assigning material properties to the two-dimensional concrete numerical model through the preset rules, then deleting the damaged and failed parts based on the failure criterion, calculating the spalling layer limit according to the coordinates and quantity of the damaged and failed parts, and performing mesh division on the processed two-dimensional concrete numerical model;

[0011] Calculating the temperature field based on the environmental temperature and the two-dimensional concrete numerical model with mesh division to obtain the temperature field; calculating the thermal stress according to the temperature change of the temperature field and establishing the stress field;

[0012] Performing a full-process numerical simulation on the two-dimensional concrete numerical model after deleting the damaged and failed parts according to the spalling layer limit, temperature field and stress field to obtain the maximum number of freeze-thaw cycles;

[0013] Predicting the life of freeze-thaw concrete based on the maximum number of freeze-thaw cycles, the amplitude and rate of environmental temperature change in the predicted area.

[0014] Preferably, the concrete material parameters include: the size of the concrete specimen, the type, gradation, and volume fraction of the aggregate, the compressive strength, tensile strength, elastic modulus, thermal conductivity, and surface heat dissipation coefficient of the mortar and the aggregate, and the elastic modulus, thermal expansion coefficient, specific heat capacity, and thermal conductivity of the pores.

[0015] Preferably, the establishment of the two-dimensional concrete numerical model specifically includes: combining the concrete material parameters with the Monte Carlo method, and establishing the aggregate, pores, ITZ, and mortar and randomly placing them by writing Python language; during the placement process, determining whether there is interference between the aggregate and the pores until the placement is successful and the expected aggregate volume fraction is reached; using the Boolean function to cut out the unfilled space of the geometric model and filling it with mortar to establish the two-dimensional concrete numerical model.

[0016] Preferably, the equivalent expansion coefficient of the pores varying with temperature given in the material property assignment is specifically expressed as follows:

[0017]

[0018] where N is the number of freeze-thaw cycles; E is the expansion coefficient of the pores, θ is the current temperature; and θ0 is the reference temperature.

[0019] Preferably, the failure criterion specifically includes: when the equivalent plastic tensile strain of the concrete exceeds the critical value corresponding to the peak tensile stress; when the equivalent plastic compressive strain of the concrete exceeds the preset threshold corresponding to the process of the peak stress dropping to the 40% level.

[0020] Preferably, in obtaining the temperature field and establishing the stress field, the first-order three-node linear heat transfer triangular element under heat transfer is adopted for the temperature field; the first-order plane stress element, that is, the three-node linear plane strain triangular element, is adopted for the stress field.

[0021] Preferably, the temperature field takes 4 hours as the period of one freeze-thaw cycle, and in the form of a cosine function, a periodically changing temperature is applied to the model, and the temperature range is +7 to -17 °C.

[0022] It can be seen from the above technical solutions that, compared with the prior art, the present invention discloses a numerical simulation method for predicting the freeze-thaw concrete life based on the spalling layer depth, establishes a two-dimensional concrete numerical model of the influence of concrete strength grade, air content and prestress magnitude on concrete freeze-thaw durability, can consider the time-varying characteristics of the performance of each component of the concrete, and aims at the pain points of the long period and high economic cost of the freeze-thaw physical test, overcomes the main technical bottleneck of the homogenization hypothesis of the existing numerical simulation technology, can provide ideas for solving the current concrete freeze-thaw durability design and evaluation methods; can more accurately predict the location of freeze-thaw concrete damage and the crack development mode, can provide an effective tool for concrete life prediction, and provide reasonable suggestions for establishing and improving the concrete durability design; starting from the freeze-thaw failure mechanism, it focuses on considering the hydrostatic pressure generated by the freezing and expansion of pore water in the concrete specimen, and combines the amplitude and rate of environmental temperature change in different regions to more accurately predict the freeze-thaw durability degradation law of concrete under different environmental action levels in different regions, and proposes a life prediction model to provide a basis for actual durability design. Description of the Drawings

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0024] Figure 1 It is the flow chart of the numerical simulation of concrete durability under freeze-thaw cycles provided by the present invention based on the equivalent temperature method and the heat convection theory;

[0025] Figure 2 It is the two-dimensional mesoscopic numerical simulation model diagram and its mesh division schematic diagram provided by the present invention;

[0026] Figure 3 Schematic diagram of the magnitude of the temperature load varying with time consistent with the laboratory loading conditions provided by the present invention;

[0027] Figure 4(a) is the temperature distribution diagram within the concrete provided by the present invention at 1 h;

[0028] Figure 4(b) is the temperature distribution diagram within the concrete provided by the present invention at 2 h;

[0029] Figure 4(c) is the temperature distribution diagram within the concrete provided by the present invention at 3 h;

[0030] Figure 4(d) is the temperature distribution diagram within the concrete provided by the present invention at 4 h;

[0031] Figure 4(e) is the temperature rise curve diagram of the center and surface of the concrete provided by the present invention;

[0032] Figure 5 Schematic diagram of the development of concrete damage with the increase in the number of freeze-thaw cycles provided by the present invention;

[0033] Figure 6(a) is the first verification comparison diagram of the spalling layer depth and relative dynamic modulus provided by the present invention;

[0034] Figure 6(b) is the second verification comparison diagram of the spalling layer depth and relative dynamic modulus provided by the present invention;

[0035] Figure 6(c) is the third verification comparison diagram of the spalling layer depth and relative dynamic modulus provided by the present invention;

[0036] Figure 7(a) is the result diagram of the influence of the concrete strength grade on the freeze-thaw durability of concrete provided by the present invention;

[0037] Figure 7(b) is the result diagram of the influence of the air content of C40 concrete on the freeze-thaw durability of concrete provided by the present invention;

[0038] Figure 7(c) is the result diagram of the influence of the air content of C45 concrete on the freeze-thaw durability of concrete provided by the present invention;

[0039] Figure 7(d) is the result diagram of the influence of prestress on the freeze-thaw durability of concrete provided by the present invention;

[0040] Figure 8(a) is the concrete life prediction diagram with a 30 mm cover thickness under the D-1 environmental action level provided by the present invention;

[0041] Figure 8(b) is the concrete life prediction diagram with a 35 mm cover thickness under the D-1 environmental action level provided by the present invention;

[0042] Figure 9(a) is the concrete life prediction diagram with a 30 mm cover thickness under the D-2 environmental action level provided by the present invention;

[0043] Figure 9(b) is the concrete life prediction diagram with a 35mm protective layer thickness under the D-2 environmental action level provided by the present invention;

[0044] Figure 10(a) is the concrete life prediction diagram with a 30mm protective layer thickness under the D-3 environmental action level provided by the present invention;

[0045] Figure 10(b) is the concrete life prediction diagram with a 35mm protective layer thickness under the D-3 environmental action level provided by the present invention;

[0046] Figure 11 It is a schematic diagram of the prestressed loading method provided by the present invention;

[0047] Figure 12 It is a flowchart of the overall life prediction model provided by the present invention. Specific Embodiments

[0048] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0049] The embodiment of the present invention discloses a numerical simulation method for predicting the life of freeze-thaw concrete based on the depth of the spalling layer. Based on the heat transfer and heat exchange principle, pore expansion theory, and equivalent temperature theory inside the concrete under freeze-thaw cycles, a numerical simulation method for predicting the freeze-thaw durability of concrete is provided, as Figure 12 shown, including:

[0050] Obtain concrete material parameters based on the concrete material using preset rules, and establish a two-dimensional concrete numerical model through the concrete material parameters;

[0051] Assign material properties to the two-dimensional concrete numerical model through the preset rules, then delete the damaged and failed parts based on the failure criterion, calculate the spalling layer limit value according to the coordinates and quantity of the damaged and failed parts, and perform mesh division on the processed two-dimensional concrete numerical model;

[0052] Calculate the temperature field based on the environmental temperature and the two-dimensional concrete numerical model with mesh division to obtain the temperature field; calculate the thermal stress according to the temperature change of the temperature field and establish the stress field;

[0053] Perform a full-process numerical simulation on the two-dimensional concrete numerical model after deleting the damaged and failed parts according to the spalling layer limit value, temperature field, and stress field to obtain the maximum number of freeze-thaw cycles;

[0054] Predict the service life of frozen-thawed concrete based on the maximum number of freeze-thaw cycles, the amplitude and rate of environmental temperature change in the predicted area.

[0055] Predict the service life of frozen-thawed concrete based on the degree of concrete damage after different numbers of freeze-thaw cycles, the average annual number of freeze-thaw cycles in the predicted area, and the environmental cooling rate; among them, estimate the average annual number of freeze-thaw cycles in the area based on the average temperature of the coldest month in different regions, and predict the service life of concrete based on the damage ratio relationship of frozen-thawed concrete under indoor laboratory conditions and on-site natural conditions established with the environmental cooling rate as a bridge.

[0056] Among them, the two-dimensional concrete numerical model includes four phases: mortar, ITZ, aggregate, and pores. Based on the pore expansion theory, a thermal expansion coefficient is assigned to the pores, and it expands and contracts continuously with temperature changes to simulate the freeze-thaw cycle process of pore water inside the concrete.

[0057] In a specific embodiment, concrete material parameters are obtained for concrete materials using preset rules. Based on the requirements of the rapid freeze-thaw method for concrete laboratories specified in the standard GB / T 50082-2009 "Test Methods for Long-Term Performance and Durability of Ordinary Concrete" and the relevant data of each mesoscopic part and freeze-thaw cycles, read the relevant parameters of each mesoscopic part of the concrete, including the size of the concrete specimen, the type, gradation, and volume fraction of the aggregate, the compressive strength, tensile strength, elastic modulus, thermal expansion coefficient, thermal conductivity, surface heat dissipation coefficient, and specific heat capacity of the mortar and aggregate. Use the Fuller gradation curve to determine the aggregate gradation and volume fraction; determine the heat dissipation coefficient on the surface of the specimen according to the convective heat transfer coefficient on the surface of the concrete; adopt the mechanical parameters (strength, elastic modulus) and thermal parameters (specific heat capacity, thermal conductivity) of the concrete specimens obtained from the tests.

[0058] In a specific embodiment, when assigning material properties to the two-dimensional concrete numerical model through preset rules, consider the water-ice phase change of the water in the concrete pores under the action of freeze-thaw cycles, which will cause the pore volume to expand and result in concrete damage. Conduct a systematic study on the pore expansion behavior of concrete under the action of freeze-thaw cycles and compare it with the results obtained from existing tests. Through multiple attempts and analyses, a relatively rigorous pore expansion coefficient α function is finally obtained, ensuring the scientificity and practicality of the expansion coefficient function in this invention.

[0059] Delete the damaged and failed parts according to the failure criterion, and call the subroutine VUSDFLD to delete the damaged elements. To more accurately reflect the spalling situation of the concrete, use the VUSDFLD subroutine to define and update the state variables of the material, and then control the failure mechanism of the material, and delete the failed elements as the spalling layer.

[0060] The ABAQUS user subroutine VUSDFLD (User subroutine toredefine field variables at a material point) was programmed using Fortran language to improve the concrete damage plasticity model and achieve the deletion of damaged elements.

[0061] In this invention, the depth of the spalled layer is used to reflect the degree of freeze-thaw damage of concrete, that is, the depth of the shedding of the surface cement mortar of the concrete member caused by freeze-thaw cycles. In numerical simulation, damaged concrete elements are deleted and regarded as the spalled layer of concrete. The depth X of the spalled layer is calculated according to the coordinates and quantity of the damaged elements:

[0062] X = d FT ×(1 + d)

[0063] In the formula: d FT is the average value of the average spalling depth of all surfaces of the concrete, and d is the correction factor. They are calculated respectively according to the following formulas:

[0064]

[0065] In the formula: x i 、y i are the horizontal and vertical coordinates of each spalled element; N FT is the number of spalled elements, and N is the total number of elements of the concrete mortar and ITZ.

[0066] By comparing the numerical simulation with the existing experiments, the limit value of the concrete spalled layer was determined:

[0067] [X] = 30% × c

[0068] In the formula: c is the concrete cover thickness.

[0069] According to the limit value of the concrete spalled layer, the maximum number of freeze-thaw cycles of the concrete was determined. Combining with the amplitude and rate of environmental temperature change in different regions, the degradation law of concrete freeze-thaw durability under different environmental action grades in different regions was predicted more accurately, and a life prediction model was proposed to provide a basis for actual durability design.

[0070] The two-dimensional concrete numerical model is based on the ABAQUS numerical simulation software. As Figure 1 shown, the calculation process is divided into two stages, namely the calculation of the temperature field and the stress field, and the sequential thermal-mechanical coupling method is adopted. In the first stage, the first-order three-node linear heat transfer triangular element (DC2D3) under heat transfer is used; in the second stage, the first-order plane stress element, that is, the three-node linear plane strain triangular element (CPE3) is used.

[0071] Among them, the ambient temperature is also obtained from the Code for Test Methods of Long-Term Performance and Durability of Ordinary Concrete (GB / T 50082-2009), which defines the load input and the external ambient temperature field in the two-dimensional concrete numerical model. In the first stage, the temperature is used as the load input for the concrete material to ensure that the temperature curve of the two-dimensional concrete numerical model is the same as the test temperature curve; in the second stage, the calculation result of the temperature field is used as a predefined field and imported into the stress field.

[0072] In a specific embodiment, the limit value of the concrete spalling layer is obtained in the post-processing stage of the numerical simulation. The concrete specimens after different numbers of freeze-thaw cycles are sorted out. The development law of the spalling layer depth is extracted, the limit value of the concrete spalling layer is proposed, and it is compared with the relative dynamic elastic modulus obtained from the physical experiment to verify the rationality of the spalling layer limit value, and a failure judgment method based on the limit value of the concrete spalling layer is proposed.

[0073] In a specific embodiment, it also includes extracting the concrete spalling layer according to the concrete damage degree to obtain the limit value of the concrete spalling layer; comparing the limit value of the concrete spalling layer with the relative dynamic elastic modulus obtained from the physical experiment to verify the effectiveness of the two-dimensional concrete numerical model; and at the same time considering the influence of preset influencing factors (including concrete strength grade, air content, initial crack characteristics and prestress magnitude) on the two-dimensional concrete numerical model.

[0074] In a specific embodiment, the concrete material parameters include: the size of the concrete specimen, the type, gradation, and volume fraction of the aggregate, the compressive strength, tensile strength, elastic modulus, thermal conductivity, and surface heat dissipation coefficient of the mortar and the aggregate, and the elastic modulus, thermal expansion coefficient, specific heat capacity, and thermal conductivity of the pores.

[0075] In a specific embodiment, establishing the two-dimensional concrete numerical model specifically includes: combining the concrete material parameters with the Monte Carlo method, and through programming in Python language, establishing the aggregate, pores, ITZ, and mortar and randomly placing them; during the placement process, determining whether there is interference between the aggregate and the pores until the placement is successful and the expected aggregate volume fraction is reached; using the Boolean function to cut out the unfilled space of the geometric model and filling it with mortar to establish the two-dimensional concrete numerical model.

[0076] 1) ITZ generation: Randomly select a point in space as the center of a circle, and generate spherical ITZs with the required particle size (aggregate particle size + ITZ thickness). Subsequently, based on the Monte Carlo method, randomly distribute solid ITZs in the space within the specimen size range in the order of decreasing ITZ particle size. During the distribution process, interference judgment needs to be performed on the ITZs to be pre-distributed until the distribution is successful. 2) Aggregate generation: Generate circular aggregates with the required particle size (aggregate particle size). On the basis of the already distributed ITZs, re-distribute the aggregates with the corresponding particle size, and use Boolean functions to cut out the ITZs and aggregates. 3) Void generation: Generate circular voids with the required particle size (aggregate particle size), distribute the voids and perform interference judgment. 4) Use Boolean functions to cut out the unfilled space of the geometric model and fill it with mortar to form a two-dimensional concrete numerical model as Figure 2 shown.

[0077] In a specific embodiment, the present invention adopts a rigorous scientific method to systematically study the pore expansion behavior of concrete under freeze-thaw cycles, and compares it with the results obtained from existing tests. Through multiple attempts and analyses, a relatively rigorous pore expansion coefficient α function is finally obtained, ensuring the scientificity and practicality of the expansion coefficient function. As the formula:

[0078]

[0079] where N is the number of freeze-thaw cycles; E is the expansion coefficient of the pores. When freezing, it is considered that the pores are filled with ice, so the elastic modulus of ice is taken to be about 500 Mpa; θ is the current temperature; θ0 is the reference temperature during the calculation by the ABAQUS software, generally defaulted to 0.

[0080] In a specific embodiment, for concrete, its damage and failure mainly fall into two cases: tensile cracking and compressive crushing. When concrete is subjected to uniaxial tensile action, once the tensile stress reaches its peak stress level, the material is considered to start to fail; while in the uniaxial compression state, after exceeding the peak pressure, concrete does not immediately lose all its bearing capacity. When the compressive stress exceeds the peak and drops to about 40% of the peak stress, the material can be regarded as completely failed. Based on this, in the VUSDFLD user subroutine, two failure criteria are set, including: when the equivalent plastic strain in tension of concrete exceeds the critical value corresponding to the peak tensile stress; when the equivalent plastic strain in compression of concrete exceeds the preset threshold corresponding to the process of the stress dropping from the peak stress to the 40% level.

[0081] In a specific embodiment, in obtaining the temperature field and establishing the stress field, the temperature field uses first-order three-node linear heat transfer triangular elements under heat transfer; the stress field uses first-order plane stress elements, that is, three-node linear plane strain triangular elements.

[0082] In a specific embodiment, in the temperature field simulation, based on the rapid freezing method in the laboratory specified in GB / T 50082, the cycle of freeze-thaw cycle is 4 hours. Therefore, in the form of a cosine function, a periodically varying temperature (+7~-17°C) is applied to the model to simulate the temperature change inside the concrete under freeze-thaw action. The schematic diagram of the temperature change during the freeze-thaw cycle is as Figure 3 shown.

[0083] The temperature field and stress field are calculated, where Figure 4(a) - Figure 4(d) is the temperature distribution diagram inside the concrete; Fig. 4(e) is the heating curve of the center and surface of the concrete; Figure 5 shows the damage development process of the concrete under freeze-thaw cycle. It truly and continuously shows the damage development inside the concrete under freeze-thaw cycle.

[0084] In a specific embodiment, the influencing factors of the freeze-thaw durability of concrete specifically include the concrete strength grade, air content, initial crack characteristics and prestress magnitude.

[0085] Figure 6(a) - Figure 6(c) The comparison of the relative dynamic elastic modulus between 3 numerical simulations and physical test results is respectively shown. When the concrete reaches the spalling layer limit value (30% of the cover thickness, 9 mm), the number of freeze-thaw cycles corresponding to the relative dynamic elastic modulus dropping to 60% of the original is basically the same, verifying the rationality of the spalling layer limit value, and a failure judgment method based on the spalling layer limit value of the concrete is proposed. The numerical simulation results are sorted and summarized. Fig. 7(a) shows the concrete strength grade, Fig. 7(b) and Fig. 7(c) show the air-entraining agent, and Fig. 7(d) shows the influence of the prestress magnitude on the freeze-thaw durability of the concrete.

[0086] Prestress:

[0087] In the numerical simulation, a reference point RP-1 is set on the top surface of the concrete, and the top surface is coupled to this reference point, and a vertically downward pressure is applied to the reference point to simulate the prestress level. The loading method is as Figure 11 shown. According to the Specification for Design of Prestressed Concrete Structures JGJ369-2016, the prestress level of the concrete structure should not exceed 0.6 times of its strength standard value f tpk . Taking C40 concrete as an example, compressive stresses of 0.10f ck , 0.20f ck , 0.30f ck and 0.40f ck are respectively applied.

[0088] Air content (air-entraining agent):

[0089] Existing research shows that the air-entraining agent dosage has the greatest impact on the frost resistance of concrete. The addition of the air-entraining agent creates a certain proportion of harmless pores inside the concrete, significantly reducing the number of harmful pores and remarkably improving the frost resistance. However, the addition of the air-entraining agent deteriorates the compressive strength and fluidity of the concrete. When the air content is 4.6 - 6.5%, the concrete has the best freeze-thaw resistance. In NB / T 20549-2019 "Durability Design Code for Nuclear Safety-related Concrete Structures", the air content of concrete is recommended to be determined according to Table 1 under different environments.

[0090] Table 1 Minimum limit requirements for air content of concrete

[0091]

[0092] From the perspective of the force-bearing of the pore structure, existing tests have proven that the air-entraining agent reduces the frost heaving force generated by the freezing of pore water in concrete, and through experimental research, it has been verified that the air-entraining agent can affect its frost resistance durability level by changing the force-bearing characteristics of the internal pore structure of the cement mortar.

[0093] Therefore, considering the influence of the air content of concrete on frost resistance, since the expansion coefficients of concrete with different air contents are different. To ensure the comparability of the calculation results, the strength grades of concrete models with different air contents should be the same in numerical simulation.

[0094] Expansion coefficient:

[0095] No air entrainment:

[0096] 4% air content:

[0097] 5% air content:

[0098] 6% air content:

[0099] The concrete strength grades are shown in Table 2:

[0100] Table 2 Mechanical parameters of the model

[0101]

[0102]

[0103] Note: The mechanical parameters of the mortar and ITZ models are the CDP model parameters of C40 concrete specified in GB / T - 50010.

[0104] For C45, C50, and C60 concrete: the elastic moduli of the mortar are 33657.4, 34500, and 36000 respectively; ITZ takes 75% of the mortar, N / mm 2 .

[0105] In the present invention, the maximum freeze-thaw cycles of concrete are determined in the two-dimensional concrete numerical model of ABAQUS by combining the concrete strength grade, air content, initial crack characteristics, and prestress magnitude with the concrete spalling layer limit, and then the concrete life prediction under different working conditions is carried out by combining the environmental temperature change range and rate in different regions.

[0106] In a specific embodiment, for the life prediction of concrete under different working conditions, the annual average freeze-thaw cycles in a place are estimated based on the average temperature of the coldest month in different regions. Based on the damage ratio relationship of freeze-thawed concrete between the indoor laboratory conditions established with the environmental cooling rate as the bridge and the on-site natural conditions, the durable life of concrete is predicted. The specific prediction model is as follows:

[0107]

[0108] In the formula: T L is the average temperature of the coldest month.

[0109] n eq = Kn act / S

[0110] In the formula: K is the saturated water time proportion coefficient of concrete during freeze-thaw cycles. For structures in frequent contact with water, it is approximately considered as 1; S is the concrete freeze-thaw damage proportion coefficient under indoor and outdoor freeze-thaw environments. Using the average cooling rate in the coldest month (January) on-site to represent the cooling rate of the on-site freeze-thaw environment, we can obtain According to the rapid freeze method in the laboratory, the indoor cooling rate can be taken as 12.5 °C / h. The highest temperature in a day appears at 14:00, and the lowest temperature is at 2:00 in the early morning. Therefore

[0111] F = n eq ·t

[0112] In the formula: F is the maximum freeze-thaw cycles of the concrete material; n eq is the indoor equivalent freeze-thaw cycles; t is the design service life. Figures 8(a) and 8(b), Figures 9(a) and 9(b), and Figures 10(a) and 10(b) show the life prediction results of concrete with different environmental action grades and different cover thicknesses under freeze-thaw cycles.

[0113] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. For the same and similar parts among the embodiments, reference can be made to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and reference can be made to the description in the method part for related parts.

[0114] The foregoing description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A numerical simulation method for predicting the life of freeze-thaw concrete based on the depth of spalling layer, characterized in that: include: Acquire concrete material parameters based on the concrete material by using preset rules, and establish a two-dimensional concrete numerical model by using the concrete material parameters; The material properties of the two-dimensional concrete numerical model are assigned according to the preset rules, and then the damaged and failed parts are deleted based on the failure criterion, the spalling layer limit is calculated according to the coordinates and quantity of the damaged and failed parts, and the processed two-dimensional concrete numerical model is meshed; The temperature field is calculated based on the two-dimensional concrete numerical model of the ambient temperature and the grid division to obtain the temperature field; the thermal stress is calculated according to the temperature change of the temperature field to establish the stress field; According to the spalling layer limit, temperature field and stress field, a full-process numerical simulation of the two-dimensional concrete numerical model after the damaged failure part is deleted is performed to obtain the maximum freeze-thaw times; The life of freeze-thaw concrete is predicted based on the maximum freeze-thaw times, the predicted range and rate of change of ambient temperature in the area.

2. A numerical simulation method for predicting freeze-thaw concrete life based on spalling depth according to claim 1, characterized in that: The concrete material parameters include: concrete specimen size, aggregate type, gradation, volume fraction, compressive strength, tensile strength, elastic modulus, thermal conductivity, surface heat dissipation coefficient of mortar and aggregate, elastic modulus, thermal expansion coefficient, specific heat capacity and thermal conductivity of pores.

3. The numerical simulation method for predicting freeze-thaw concrete life based on spalling depth according to claim 1 is characterized in that: The establishment of the two-dimensional concrete numerical model specifically includes: according to the concrete material parameters combined with the Monte Carlo method, by writing Python language, establishing aggregates, pores, ITZ and mortar and randomly placing them; during the placing process, judging whether there is interference between aggregates and pores until the placing is successful and the expected aggregate volume fraction is reached; using Boolean functions to cut out the unfilled space of the geometric model and fill it with mortar to establish the two-dimensional concrete numerical model.

4. The numerical simulation method for predicting freeze-thaw concrete life based on spalling depth according to claim 1 is characterized in that: The specific expression of the pore equivalent expansion coefficient that varies with temperature in the material property assignment is as follows: Where N is the number of freeze-thaw cycles; E is the expansion coefficient of the pores, θ is the current temperature; and θ0 is the reference temperature.

5. The numerical simulation method for predicting freeze-thaw concrete life based on spalling depth according to claim 4 is characterized in that: The failure criteria specifically include: when the tensile equivalent plastic strain of the concrete exceeds the critical value corresponding to the peak tensile stress; when the compressive equivalent plastic strain of the concrete exceeds the preset threshold corresponding to the process of decreasing from the peak stress to the 40% level.

6. The numerical simulation method for predicting freeze-thaw concrete life based on spalling depth according to claim 1 is characterized in that: In obtaining the temperature field and establishing the stress field, the temperature field adopts the first-order three-node linear heat transfer triangle unit under heat transfer; the stress field adopts the first-order plane stress unit, that is, the three-node linear plane strain triangle unit.

7. A numerical simulation method for predicting freeze-thaw concrete life based on spalling depth according to claim 6, characterized in that: The temperature field takes 4 hours as a freeze-thaw cycle, and uses a cosine function to apply a periodically changing temperature to the model, with a temperature range of +7 to -17°C.

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